A modified RS-485 serial communication bus, such as the IntelliBus™, for example, has a transmission line length of up to approximately 6,000 feet. Serial communication buses of this type transmit a differential signal, are topology independent (i.e., all topologies of wiring are compatible), do not require termination at either the transmitting or the receiving ends and have a maximum data transmission rate of 38.4 KBaud.
Typically, the transmitted differential signal travels along the transmission line at 0.15 nanoseconds per foot. The transmitted signal reaches the end of the transmission line and is reflected back before the end of the rise or fall time of the originally transmitted signal. This signal reflection disrupts the transmitted signal. To compensate for the signal reflection, the rise and fall times of the transmitted signal are altered by adjusting the RC time constant of the transmitting circuit (i.e., control the slew rate of the transmitted signal). However, adjusting the RC time constant results in a portion of the high frequency component of the transmitted signal being filtered (lost) and the maximum data transmission rate being limited to 38.4 KBaud. Further, the transmission line also acts as a low-pass filter, which further filters the high frequency component of the transmitted signal. As the transmission line length increases, more of the high frequency component of the transmitted signal is lost. Thus, any data in the high frequency portion of the transmitted signal is also lost. The maximum transmission line length is therefore determined by the maximum allowable loss of the high frequency component (of the transmitted signal) that will still allow the data to be received. Currently, the transmission line length of a modified RS-485 serial communication bus, such as the IntelliBus™, is limited to approximately 6,000 feet.